1. Field of the Invention
The present invention relates to a distance measurement device and a distance measurement system each including a photosensor, and particularly to a distance measurement device and a distance measurement system each employing a time-of-flight (TOF) method.
2. Description of the Related Art
There has been increasing interest in three-dimensional distance measurement systems that can detect a distance from a light source to an object with a detection signal dependent on delay in light travel time. Three-dimensional distance measurement systems are expected to have a wide range of applications such as virtual keyboards, information input devices with a gesture recognition function and the like, visual sensors in various robots, security systems, sensors for smart air bags, and sensors for cars.
A TOF method is known as one of methods for measuring a distance. A distance measurement system employing a TOF method obtains a distance between a light source (the distance measurement system) and an object by calculation in such a manner that light is emitted from the light source to the object and, when light reflected by the object reaches a sensor, light travel time delay that occurs between the emitted light and the reflected light is detected. Specifically, a distance x from the light source to the object can be represented by the following formula using light speed c and delay time Δt.
  x  =            c      ×      Δ      ⁢                          ⁢      t        2  
Non-Patent Document 1 discloses a three-dimensional image sensor that measures a distance in a three-dimensional space by performing three-dimensional imaging with a TOF method in such a manner that a period for detecting reflected infrared light is divided into two periods to obtain different detection signals.
In Non-Patent Document 2, two-dimensional imaging and three-dimensional imaging are alternately performed every frame period with the use of the configuration of the sensor in Non-Patent Document 1.